As is known, a belt-type continuously variable transmission has been used as a transmission for a vehicle. Such a continuously variable transmission includes a drive pulley 100 and a driven pulley 101, each of which is capable of changing the width, and a metal belt 102 wound around the pulleys. When the width of the drive pulley 100 and the driven pulley 101 is changed and the winding radius of the belt 102 thereon is changed, continuous gear shifting can be performed. The width of the pulleys 100 and 101 in the belt-type continuously variable transmission is changed through adjustment of oil or hydraulic pressures, so called sheave pressures Pin, Pout, applied to cylinder chambers 103 and 104, which are provided for the pulleys 100 and 101, respectively. Therefore, a hydraulic control system 110 for controlling the hydraulic pressures for adjusting sheave pressures is provided in the belt-type continuously variable transmission.
A hydraulic control system described in Japanese Laid-open Patent Publication 11-247981 is known as a hydraulic control system for the belt-type continuously variable transmission. In the hydraulic control system described in this document, as illustrated in FIG. 2, the pressure of the operating oil supplied from a pump is adjusted by a first regulator valve 111 and a second regulator valve 112, thereby generating line pressure Pl serving as a source control pressure. The line pressure Pl is decreased by a modulator valve 113, thereby generating modulator pressure Pm which is constant.
The modulator pressure Pm output from the modulator valve 113 is supplied to a first solenoid valve 114 and a solenoid valve 115. The first and second solenoid valves 114 and 115 adjust the modulator pressure Pm through power-distribution control of their built-in linear solenoids to generate and output desired first solenoid pressure Pslp and second solenoid pressure Psls, respectively.
The first solenoid pressure Pslp output from the first solenoid valve 114 is transmitted to a first sheave pressure adjustment valve 116. Then, the first sheave pressure adjustment valve 116 adjusts the line pressure Pl in accordance with the first solenoid pressure Pslp to generate a first sheave pressure Pin that is to be applied to the drive pulley. On the other hand, the second solenoid pressure Psls output from the second solenoid valve 115 is transmitted to a second sheave pressure adjustment valve 117. Then, the second sheave pressure adjustment valve 117 adjusts the line pressure Pl in accordance with the second solenoid pressure Psls to generate a second sheave pressure Pout that is to be applied to the driven pulley. In this manner, in the hydraulic control system, gear shift of the transmission is controlled by controlling the first and second sheave pressures Pin and Pout applied to the drive pulley and the driven pulley, respectively, by power distribution control of the linear solenoids of the first and second solenoid valves 114 and 115 so as to variably set the width of the pulleys.
In the hydraulic control system of JP-A-11-247981, the first and second solenoid pressures Pslp and Psls output from the first and second solenoid valves 114 and 115 are also transmitted to the second regulator valve 112. The second regulator valve 112 uses the higher solenoid pressure of the supplied first and second solenoid pressures Pslp and Psls to generate and output line pressure control oil pressure Psrv. Then, the first regulator valve 111 adjusts the line pressure Pl in accordance with the line pressure control oil pressure Psrv. In other words, in the hydraulic control system, the line pressure Pl is adjusted in accordance with the higher solenoid pressure of the first and second solenoid pressures Pslp and Psls.
The first sheave pressures Pin and the second sheave pressures Pout in the hydraulic control system are represented by the following equations (1) and (2), respectively. The line pressure Pl is represented by the following equation (3).Pin=α*Pslp+β1  (1)Pout=α*Psls+β2  (2)Pl=α*MAX(Pslp,Psls)+β  (3)wherein α is a gain of the sheave pressure with respect to the solenoid pressure and β1, β2, and β are constants determined depending on the mechanical configurations, dimensions, and spring loads of the valves.
FIG. 3 illustrates the relation between the change gear ratio γ and the hydraulic pressures in the hydraulic control system of JP-A-11-247981. The line pressure Pl is set to pressure that is slightly higher than the higher sheave pressure of the first sheave pressure Pin and second sheave pressure Pout. Therefore, the line pressure Pl is suppressed to the requisite minimum.
In this manner, in the conventional hydraulic control system for the transmission, the line pressure Pl can be suppressed to the requisite minimum as well as reduction in fuel the mileage and increase in the oil temperature can be suppressed. However, the conventional hydraulic control system for the transmission has the problems as described below and still has room for improvement.
Specifically, in the hydraulic control system having the configuration in which the line pressure Pl is set by using the higher solenoid pressure of the first solenoid pressure Pslp and the second solenoid pressure Psls, the gains of the sheave pressures with respect to the solenoid pressures are required to be equal on the drive side and on the driven side, which limits the degree of freedom of hydraulic control.
FIG. 4 illustrates the relation between the change gear ratio γ and the hydraulic pressures in the case where the gain η of the first sheave pressure Pin with respect to the first solenoid pressure Pslp at the first sheave pressure adjustment valve 116 and the gain α of the second sheave pressure Pout with respect to the second solenoid pressure Psls at the second sheave pressure adjustment valve 117 are different from each other. The gain η is set to be smaller than the gain α (η<α). In this case, in the region where the line pressure Pl is set in accordance with the first solenoid pressure Pslp, i.e., the region in which the first solenoid pressure Pslp is higher than the second solenoid pressure Psls, the line pressure Pl becomes significantly higher than the necessary oil pressure (first sheave pressure Pin). In other words, in such a region, the line pressure Pl is set excessively high. In order to suppress the line pressure Pl to the requisite minimum, the gain (η) of the drive side and the gain (α) of the driven side have to be equal.
In many belt-type continuously variable transmissions, the cylinder area of the drive side is larger than the cylinder area of the driven side in order to suppress the sheave pressure to a low level in the range of change gear ratio on the accelerating side. In this case, the maximum value of the first sheave pressure Pin can be normally made lower than the maximum value of the second sheave pressure Pout. However, in the conventional hydraulic control system, the gain on the drive side and the gain on the driven side have to be equal. Accordingly, if the maximum pressures of the first and second solenoid pressures Pslp and Psls are the same, the maximum pressure of the first sheave pressure Pin becomes significantly higher than it is required to be under normal circumstances. Therefore, when the first solenoid pressure Pslp increases to near the maximum pressure due to surge pressure upon failure or starting of the first solenoid valve 114, excessive thrust force is applied to the drive pulley thereby lowering the durability of the belt.
When the cylinder area of the drive side is made larger than the cylinder area of the driven side, the gain of the drive side can be normally reduced more. However, in such a case, the gain of the drive side is set to the value exceeding that is required under the normal circumstances so that the gain of the drive side and the gain of the driven side are made to be equal. When the gain of the sheave pressure with respect to the solenoid pressure is increased, an error in the solenoid pressure is amplified more greatly and affects the sheave pressure. Thus, controllability is reduced, and variation in the sheave pressure increases. Therefore, in spite of the fact that the gain can be suppressed to be smaller under normal circumstances in the above case, the variation in the sheave pressure (first sheave pressure Pin) of the drive side becomes unnecessarily large. When such variation in the sheave pressure is increased, the line pressure Pl has to be set higher in order to provide allowance with respect to the variation, which impairs the effect of improving mileage.
As described above, in the conventional hydraulic control system for the transmission, the gains of the sheave pressures with respect to the solenoid pressures have to be made equal on the drive side and the driven side whereby optimization of the sheave pressure control is inhibited. This causes troubles such as reduction of the durability of the belt of the continuously variable transmission and the limited effect of improving mileage.